This invention relates to detection and cancellation of non-repeatable run-out due to spindle motor cage vibration during a servowrite operation in a disc drive.
Spindle motor vibration, known as non-repeatable runout (NRRO), causes motion between the head and media during the rotation of the media by the spindle motor. Spindle NRRO is composed of low-frequency vibration generated by the ball bearings of the motor, the largest single component being the cage vibration generated by the motor cage. Cage vibration is an extremely low frequency vibration that causes DC track spacing errors during servo writing.
Servowrite operations are performed during manufacture of the disc drive to record servo data onto one or more disc surfaces of the disc drive. The servo writer is affixed to the disc drive and includes a read/write clock head positioned by an actuator arm over the disc to write clock patterns to the disc and to control the data head of the disc drive to write servo data patterns to the disc. In disc drives employing embedded servo patterns, the servo data patterns are written into servo sectors between user fields on each track.
Servo patterns are written on the disc by first writing a clock pattern onto the entire track, or group of tracks, on the disc, including those portions that will become user fields. The clock pattern is usually written over more than one convolution of the disc, over-writing the clock pattern recorded during the prior convolution, to assure that the clock pattern is recorded over the entire track. The end point, where the two recorded convolutions meet, is called the splice point. Next, the track is separated into servo sectors and user data fields, and the disc drive data head writes the servo pattern over the recorded clock patterns in the servo sectors while the clock head reads the clock pattern.
Radial motion of the disc beneath the data head due to spindle motor cage vibration causes the servo writer to write the servo pattern in a spiral, rather than a circle, resulting in a radial discontinuity of the servo pattern at the splice point. As a result, the servo sectors are recorded as segments of that spiral, and not truly on the circular path of the track.
Many servowriters include a laser positioning interferometer to measure relative movement between the disc drive data head and the clock head of the servo writer. However, the laser interferometer can not measure movement between the data head and the disc, which includes low-frequency motion due to cage frequency.
A high magnitude of spindle motor cage vibration in servowriter operations may cause a serious track closure or track squeeze errors, known as track misregistrations. The track closure error is evidenced by a phase error signal splice during servo pattern write operations that cause servo off-track failures during drive operations. The track squeeze is evidenced by adjacent tracks positioned closer than expected at various locations on the media. This track misregistration is a type of write-to-write track misregistration that generates data cross-talk between adjacent tracks and/or distorts the servo pattern causing defect servo pattern errors.
Experiments on motor cage vibration show that cage vibration is non-synchronous to spindle rotation, but is periodic. The periodic waveform of cage vibration repeats itself over intervals longer than one spindle rotation. Hence, a phase relationship exists between the waveform and spindle rotation. Moreover, the frequency of cage vibration is the same for all motors of same type, regardless of the disc drive system in which they are employed. Hence, the error written in the tracks due to cage vibration has a similar magnitude whether there are track closure errors or not. However, track closure and track squeeze appear to occur randomly due to complicated phase relationships of the cage frequency, spindle rotation and starting phase of the servo write.
The present invention provides a solution to this and other problems associated with cage vibration, and offers other advantages over the prior art.
The present invention is directed to a process of writing servo patterns to a storage disc of a disc drive. In one embodiment of the invention, the magnitude and frequency of runout due to vibration of the cage of the spindle motor is detected. A plurality of servo patterns are written to a reference track on the disc, and the plurality of servo patterns are then read over a plurality of revolutions of the disc using a read head, such as the read portion of a clock head. The magnitude of position error of the clock head relative to the reference track is identified for each of the read servo patterns. The magnitude and frequency of runout due to cage vibration is identified from the position errors.
In one form the invention, the servo patterns are written to user tracks on the disc by identifying periods of low magnitude runout due to cage vibration, and writing the servo patterns to the user track during those periods of low magnitude runout.
In some embodiments, the periods of low magnitude runout are identified by identifying the magnitude of a splice in the reference track and identifying periods when the magnitude of the splice is not greater than a threshold of acceptable position error.
In preferred embodiments, a track closure error is identified for the user track. If the track closure error exceeds a predetermined threshold, the writing of the servo pattern is repeated until the track closure error does not exceed the threshold.
In other preferred embodiments, track spacing between the user track and an adjacent track is identified. If the track spacing is less than a predetermined spacing threshold, writing of the servo pattern to the user track is repeated until the track spacing is not less than the spacing threshold.
Additional features and advantages will become apparent upon review of the following drawings and the accompanying detailed description.